This invention relates to an oxygen-blown steelmaking furnace into which pure oxygen is top-blown, and either nothing or one or two of oxygen, carbon dioxide and inert gas are bottom-blown.
Recently, a second look has been given to bottom-blown steelmaking furnaces, in an attempt to make up for the shortcomings of top-blown furnaces such as great oxidization-induced iron loss and poor dephosphorization. But the bottom-blown furnaces are not without problems. For example, the minimum hot-metal ratio in the charge is higher because less iron is oxidized during blowing. Also, top-blown furnaces cannot be remodelled into the bottom-blown type easily because of complex equipment requirements.
Thus, bottom- as well as top-blown furnaces, into which pure oxygen is top-blown and oxygen, carbon dioxide and/or inert gas is bottom-blown, have been proposed.
But even this last-mentioned type involves several operating problems calling for improvement.
The bottom- and top-blown furnaces now in common use are equipped with such a hot-metal temperature and carbon content control device as measures the actual temperature and carbon content of hot metal immediately, for example between 1 and 5 minutes, before the completion of blowing by use of a sublance. Then, the targeted hot metal temperature Tt and carbon content Ct are obtained by changing the level of the lance, the quantity of fluxes added, and the quantity of oxygen, carbon dioxide and inert gas blown according to the difference between the measured and targeted values.
Through the experience in operation, however, the inventors learnt that the aforementioned conventional device required improvement since it was unable to attain the targeted hot metal temperature and carbon content with high enough precision. It was also found that the conventional control device was unable to effectively control the phosphorus and manganese contents, although their variations are less than in the top-blown furnaces.
Although known as the dynamic control, this conventional technique performs static computation based on the hot-metal information collected through the measurement made by use of a sublance. It does not go as far as to determine the changes in the decarburizing and slag-making reactions during blowing. Accordingly, despite various operating efforts, the accuracy with which the targeted hot-metal temperature and carbon content are attained is not high enough. Also, the varying slag-making reactions entail considerable variations in the phosphorus and manganese contents in hot metal. These shortcomings call for improvement.
The following gives a more detailed description of the static and dynamic control mentioned before.
As stated previously, there are known steelmaking furnaces which are equipped with a device to perform static control or one to perform dynamic control, or one to perform both. Static control is a process that presets various operating conditions, such as the quantity of oxygen to be blown and that of fluxes to be added, before starting refining and completes refining according to the preset program. Meanwhile, dynamic control completes refining while modifying the operating conditions based on the dynamic information collected during the course of refining.
Various methods have been proposed for collecting the aforementioned dynamic information; such as one that analyzes the waste gas from the refining process as disclosed in the "Method of Controlling Oxygen Furnace (Japanese Patent Publication No. 23695 of 1967)" and "Method of Monitoring and Controlling in the Oxygen Top Blowing Process (Japanese Patent Publication No. 4088 of 1968)," one that uses a sublance as disclosed in the "Method of Controlling the Basic Oxygen Steelmaking Process (U.S. Pat. No. 3,574,598)," and one that combines the analysis of waste gas with the use of sublance as disclosed in the "Method of Estimating Hot-metal Temperature and Carbon Content in Oxygen Furnace (Japanese Patent Public Disclosure No. 101617 of 1977)" proposed by the inventors. Especially the last-mentioned combination method has greatly improved the precision with which the targeted temperature and carbon content of hot metal are attained at the end-point. Some other proposed methods lay emphasis on the operation of the top- and bottom-blown furnaces, such as those disclosed in the "Process and Apparatus for Making Alloy Steels (Japanese Patent Public Disclosure No. 8109 of 1976)" and "Method of Operating Oxygen Furnaces (Japanese Patent Public Disclosure No. 146711 of 1977)." But these methods involve the following problems.
Namely, even when it is expected to raise the hot-metal temperature by varying the lance level, the quantity of oxygen supply, and the decarburizing and slag-making reactions by varying the ratio of oxygen consumption therebetween through the control of carbon dioxide and inert gas supplies, the aforementioned methods do not offer any measured data by which the results of such changes can be estimated. Consequently, the change in the hot-metal temperature and carbon content near the end point can be corrected only by cooling.
In this type of practice, the quantity of flux addition is preset so that the hot-metal temperature at the end point would become slightly higher than the targeted level. Then the targeted temperature is attained by correcting the addition based on the latest information collected as the operation nears toward the end point. Accordingly, the temperature of hot metal remains higher than is desired over the greater part of the blowing period, producing a detrimental effect on the furnace refractories. Dephosphorization is one of the objects of the blowing given into the oxygen furnace. But the higher hot-metal temperature creates a metallurgical atmosphere undesirable for dephosphorization. This necessitates either adding more base or oxidizing the slag to a greater extent, but both steps are not free from quality and cost problems. In the "Method of Estimating Hot-metal Temperature and Carbon Content in Oxygen Furnace (Japanese Patent Public Disclosure No. 101617 of 1977)," mentioned previously, and the "Method of Controlling Hot-metal Temperature and Carbon Content in Oxygen Furance (Japanese Patent Public Disclosure No. 101618)," the accuracy of control is improved by continuously measuring the in-furnace distribution of oxygen, near the end point, consumed for decarburization and iron-oxidization. According to these methods, the hot-metal temperature can be lowered by adding more fluxes. The temperature can be raised by varying the lance level, the quantity of oxygen supply, and the decarburizing and slag-making reactions by varying the ratio of oxygen consumption therebetween through the control of carbon dioxide and inert gas supplies, with such changes properly measured. So it is unnecessary to make such flux addition as might raise the hot-metal temperature slightly above the targeted level, confining the method of correction to cooling. Even then, however, the changes in the hot-metal temperature and carbon content up to near the end point may possibly deviate greatly from the course in which their end-point targets can successfully be hit. Under such circumstances, the supply of oxygen, carbon dioxide and inert gas, the quantity of flux addition, and the level of the lance must be varied compensatingly. But such actions introduce a significant change in the oxygen distribution in the furnace, make the slag-making reaction unstable, and cause wide variations in the phosphorus, manganese and oxygen contents in steel, giving rise to serious cost and quality problems.
To solve these problems, as mentioned previously, it is necessary to develop a high-precision static control measure and a dynamic control measure well-matched thereto. Various static control measures have been proposed, but, to the knowledge of the inventors, none of them can ensure high-precision control.